Actinobacillus actinomycetemcomitans is the primary periodontal pathogen in localized juvenile periodontitis (LJP). These patients often exhibit elevated levels of serum antibodies towards outer membrane constituents of A. actinomycetemcomitans including endotoxin and outer membrane proteins. Recent evidence has suggested that LJP patients generate elevated levels of specific antibody to a 29 kDa outer membrane protein (OMP), which by N-terminal amino acid sequencing has been shown to be a member of the OMPA family of proteins. However, the IgG generated contains an unusually high amount of antibody of the IgG2 subclass, which is generally raised against polysaccharide antigens eg. endotoxin. While this IgG subclass generally does not confer protective immune responses, recent observations show that IgG2 from LJP sera does contain opsonic antibody, and thus may indeed be protective. It is currently not known whether specific IgG2 antibodies against the 29 kDa protein are opsonic in nature. Current efforts have been made toward molecular cloning of this 29 KDa OMP to further elucidate the nature of this altered IgG2 subclass response. Preliminary data reveal the possibility that the gene encoding the 29 kDa OMP has been cloned through polymerase chain reaction (PCR) technology utilizing PCR primers specific for the OMPA homologue in H. influenzae. Presently we are sequencing the cloned gene for the 29 kDa protein. Future studies involve production and characterization of A. actinomycetemcomitains mutants defective in OMPA. These mutants will then be used to determine if OPMA mediates serum resistances (OMPA in E.coli mediates serum resistance) and attachment to epithelial cells in vitro, both important pathogenic properties of Actinobacillus actinomycetemcomitans. Key words: Outer membrane proteins; molecular cloning; IgG subclass response: